In a conventional backlash prevention device disclosed in Japanese Utility Model Application (OPI) No. 178070/84 (the term "OPI" as used herein means an "unexamined published application"), the rotation of a spool is automatically braked when a set time has elapsed since the start of the rotation of the spool. It is difficult, however, to precisely set the time that it will take for a fishhook, a fishline, and so forth to reach an aimed point after the start of the rotation of the spool. Since the cast length of the fishline differs from time to time, the rotation of the spool is not braked if the fishhook, the fishline, and so forth reach the aimed point within a period shorter than the set time. For that reason, there is a problem that the fishline is likely to backlash.
Another backlash prevention device for solving the above-mentioned problem was disclosed in Japanese Utility Model Application (OPI) No. 1575/87. In the device, the number of pulses from a reference clock signal generation circuit to detect the rotation of the spool, and the speed and acceleration of the rotation of the spool are calculated from the number of pulses counted. The rotation of the spool is braked when the calculated speed and acceleration are equal to set values or are negative. The rotation of the spool is not braked when the calculated speed and acceleration are zero. Consequently, backlash is prevented regardless of the cast length of the fishline or the faulty casting thereof.
In still another backlash prevention device for solving the above-mentioned problem, the speed and acceleration of the rotation of a spool are calculated by a CPU in terms of a signal from a timer circuit and a signal from a sensor for detecting the rotation of the spool and are compared with reference data. The rotation of the spool is braked by causing an eddy current in a nonmagnetic electromagnetic member when the differences between the calculated speed and acceleration and the specified values are not less than prescribed values. The rotation of the spool is not braked when the differences are less than the prescribed values. As for the device, the rotation of the spool can be braked as occasion demands, to shorten the time of the braking.
Since a magnet must be rotated by a motor in each of the above-mentioned conventional backlash prevention devices so as to automatically prevent the backlash of the fishline, they have the disadvantage that the backlash cannot be prevented when a power supply (such as a battery) for the motor is not effectively usable.
In yet another backlash prevention device, a motor is rotated in accordance with a signal to turn a magnet ring so that the rotation of the spool is braked when the calculated speed and acceleration are equal to set values or are negative and the rotation of the spool is not braked when the calculated speed and acceleration are zero. Since the rotation of the motor needs to be regulated to stop the magnet ring in positions accurately corresponding to the maximum and minimum levels of the braking force on the spool, respectively, the motor is complicated and expensive. If conventional components such as a limit switch and an encoder are used as means for controlling the quantity f rotation of an ordinary motor for such a backlash prevention device, a larger installation space is needed for the device which prevents the reduction in the size and weight of a fishing reel and increases the cost of production thereof. This is still another problem.